An Amazon executive fired back at IBM–which is running a marketing campaign targeting the online retailer’s cloud computing business–marking the latest salvo in a growing war of words between the two tech giants.

Amazon Web Services chief Andy Jassy, during an address Wednesday at its annual conference, said IBM is just “jumping up and down” in an attempt to “confuse customers” about its cloud computing offerings. Earlier this month, IBM launched a marketing campaign that implicitly claims IBM is the leader over AWS in the estimated $40 billion-a-year cloud-computing market.

“We believe customers are not going to be allowed to have the wool pulled over their eyes,” said Jassy.

The spat shows how crucial cloud computing is becoming as more companies eschew the expense of buying and maintaining their own servers in favor of turning over their storage, data and other computing needs to other firms. Cloud computing is when companies share space on centralized Internet-connected servers for a fee.

“We’re not being shy about spreading the word that unlike Amazon, IBM has a long history of delivering trusted, secure and open cloud services for years,” said Nancy Pearson, vice president of IBM’s cloud marketing, in an emailed statement Wednesday following Jassy’s comments. “As more clients embrace cloud computing to transform their businesses, IBM continues to infuse high value capabilities, such as our acquisition of SoftLayer and continuous support for open cloud standards. Through this campaign, we’re taking the facts about our cloud leadership straight to customers.”

IBM began the marketing campaign, which includes print advertising and, apparently, painted trucks driving around New York, after losing out to Amazon on a $600 million cloud computing contract with the CIA. IBM had fought for the contract in court before a federal judge ruled in favor of Amazon.

IBM said in its most recent quarter it generated more than $1 billion in cloud revenue, up 70% from a year earlier, the first time IBM disclosed its cloud revenues. Amazon doesn’t disclose cloud computing revenue, but analysts estimate it could be around $3 billion a year.

Note: An earlier version of this item incorrectly reported that Jassy said IBM has a larger cloud computing business than AWS.